1. Field of the Invention
The present invention relates to solid, oxazoline-terminated, urethane-functional polyaddition compounds, to a process for preparing them and to their use for preparing plastics, especially powder coating materials, which crosslink to give high-gloss coating films which are stable to light and weathering.
2. Description of the Background
Externally or internally blocked polyisocyanates, which are solid at room temperature, are valuable crosslinking agents for thermally crosslinkable polyurethane (PU) powder coating materials. For example, DE-A 27 35 497 describes PU powder coating materials having excellent weathering and thermal stability. In another disclosure, which is DE 27 12 931, the crosslinking agents, whose preparation is described therein, consist of .epsilon.-caprolactam-blocked, isocyanate-functional isophorone diisocyanate. Also known are urethane-, biuret- and urea-functional polyisocyanates, whose isocyanate groups are likewise blocked.
The disadvantage of these systems lies in the elimination of the blocking agent during the thermal crosslinking reaction. Since the blocking agent can, therefore, be discharged into the environment, it is necessary on ecological and occupational-hygiene grounds to take special precautions to remove cleanse the atmosphere discharged from the process and to recover the blocking agent. In addition, the crosslinking agents normally employed have low reactivity, i.e., curing temperatures of more than 170.degree. C. are required.
DE 30 30 539 and DE 30 30 572 describe processes for preparing urethane-functional polyaddition compounds whose terminal isocyanate groups are capped irreversibly with monoalcohols or monoamines. The chain-terminating constituents of the crosslinking agents are particularly disadvantageous, since they lead to low network densities in the PUR powder coatings and thus to moderate solvent resistances.
Hydroxyl-terminated, uretdione-functional polyaddition compounds are the subject of EP 0 669 353. Their functionality of two accordingly constitutes an enhanced resistance to solvents. A feature common to the powder coating materials, based on these uretdione functional polyisocyanates, is that they emit no volatile compounds in the course of the curing reaction. However, at a minimum of 180.degree. C., the stoving temperatures are high.
Publications JP 51 125 116, JP 49 124 130, JP 51 054 626, DE 25 05 862 and U.S. Pat. No. 4,245,063 describe powder coating materials whose oxazoline-functional acrylate copolymers are cured with polycarboxylic acids. The oxazoline groups are incorporated into the resin by copolymerizing 2-isopropenyl-2-oxazoline with unsaturated monomers.
Powder coating materials based on acid polyesters or acrylates and polyoxazoline curing agents are described, for example, in DE 23 28 012, DE 23 28 013, JP 51-136727, JP 50-037832, JP 56-036551, JP 51-134 727 and JP 51-089534. The polyoxazoline curing agents contain no additional functional groups, and so the flexibility of the powder coatings and their adhesion to the substrate are in need of improvement.
DE 44 42 908 describes the use of oxazoline-terminated polyurethanes. These compounds, however, are liquid and are, therefore, unsuitable for use as powder coating hardeners (curing agents). Moreover, in this case curing is initiated by cationic polymerization, which is known to be highly susceptible to disruption because of the presence of impurities. Moreover, such polymerization conditions rule out the use of basic pigments and auxiliaries.